A wide variety of particulate additives are used in thermoplastic polymers in order to improve the properties of the polymer and/or the utility of products formed from such polymers. Included among the types of particulate polymer additives used are, for example, such substances as antioxidants, flame retardants, flame retardant synergists, thermal stabilizers, UV stabilizers, nucleating agents, acid neutralizers, polymer clarifiers, and the like. In order to facilitate blending operations it is desirable to provide additives in a granular form because many additives can cake up or form rat-holes in feed hoppers, and/or feed unevenly through metering equipment. Moreover, certain additives, especially finely-divided additives, often used in the manufacture of polymers, such as high melting nucleating agents and inorganic acid neutralizers can create hazardous airborne dusts during handling and blending operations.
Various methods for converting polymer additives into granular forms have been described heretofore. Such methods include melting at least one component of a dry blend of additives to thereby bond the particles of the blend together, or including in a blend of the additives a special component such as a wax, fatty acid, a compound containing a fatty acid chain or fatty alcohol chain, or metal salt of a fatty acid, and then converting the dry blend into granules or pellets by compacting or milling the blend. All such methods require use of extraneous components to serve as binders, which components are not necessarily desired as components in the finished polymer composition, and which may actually interfere with product specifications of the polymer producer. Moreover, use of some previously used binders can detract from performance properties of the host polymer. And, in some cases the scope of the additives that can be converted into granules or pellets is quite specific and thus not of widespread utility.
Some of these prior developments are described, for example, in U.S. Pat. Nos. 4,957,956; 5,240,642; 5,597,857; 5,844,042, 5,846,656; and 6,033,600.
It would be of considerable advantage if a way could be found of converting a blend of two or more particulate polymer additives into granular form such as pellets without depending upon melting a component of the blend or the binding action of a special binder component included in the powder blend such as a wax, a fatty acid, a compound containing a fatty acid chain or fatty alcohol chain, or metal salt of a fatty acid. It would be of even greater advantage if the scope of the blends of particulate additives converted into granules such as pellets could be broadened so that it is unnecessary to rely on only certain specific combinations of additives that produce granules.
This invention is deemed to accomplish these objectives in an efficient and effective manner.